This application is based on Japanese Patent Application No. 2000-46471 filed Feb. 23, 2000, the content of which is incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a sputtering target for production of a magnetic recording medium, which is used for film formation of a magnetic layer of the magnetic recording medium mounted in various magnetic recording devices, including external storage devices of computers. The invention also relates to a method for producing a magnetic recording medium by use of the sputtering target, and a magnetic recording medium produced by this method for production.
Hereinafter, the sputtering target will be referred to simply as a xe2x80x9ctargetxe2x80x9d.
2. Description of the Related Art
Various compositions and structures of a magnetic layer and various materials for a nonmagnetic undercoat layer have been proposed for a magnetic recording medium required to have a high recording density and a low noise. In recent years, a proposal has been made for a magnetic layer, generally called a granular magnetic layer, which has a structure comprising magnetic crystal grains surrounded with a nonmagnetic nonmetallic substance such as an oxide or a nitride.
The granular magnetic layer is considered to obtain low noise characteristics for the following reason: A nonmagnetic nonmetallic grain boundary phase physically separates the magnetic grains. Thus, the magnetic interaction between the magnetic grains lowers to suppress the formation of zigzag domain walls occurring in a transition region of recording bits.
Examples using the granular magnetic layer are shown in Japanese patent Application Laid-open No. 8-255342 (1996) and U.S. Pat. No. 5,679,473, Japanese Patent Application Laid-open No. 8-255342 (1996) proposed the achievement of a low noise by laminating a nonmagnetic film, a ferromagnetic film, and a nonmagnetic film sequentially and then heat treating the laminate to form a granular recording layer comprising ferromagnetic crystal grains dispersed in the nonmagnetic film. In this case, a silicon oxide or nitride is used as the nonmagnetic film.
U.S. Pat. No. 5,679,473 describes that RF sputtering is performed with the use of a CoNiPt target incorporating an oxide such as SiO2, whereby there can be formed a granular recording layer having a structure comprising magnetic crystal grains surrounded by a nonmagnetic oxide and thereby individually separated. As a result, a magnetic recording medium having high Hc (coercive force) and a low noise is obtained. The method described in U.S. Pat. No. 5,679,473, for film formation of a granular magnetic layer by performing RF sputtering using a CoNiPt target incorporating an oxide such as SiO2, can produce a magnetic recording medium without involving process such as heat treatment, and is thus excellent in mass producing ability.
However, when a target having an oxide dispersed in a metal is sputtered, abnormal discharge due to charging of oxide particles, as an insulating material, at the start of sputter discharging tends to occur. When abnormal discharge occurs, the surface of the target is partially heated, and thus melts and flies. As a result, oxide particles measuring several micrometers may adhere onto a magnetic recording medium disposed at an opposed position.
If the oxide particles thus adhere onto the magnetic recording medium, the incident that the particles collide with the head arises. To prevent this incident, the particles may be removed by sliding, for example, a cleaning tape over the medium having the film formed by sputtering. This sliding treatment peels not only the particles, but also the magnetic layer and a protective layer, causing problems such as signal defects or corrosion.
Japanese Patent 2948019 describes that the particle diameter of an insulating substance is set at 20 xcexcm or less in order to resolve abnormal discharge occurring when sputtering is performed using a sputtering target comprising a sintered mixture of the insulating substance and a conductive substance. However, this sputtering target is used for film formation of a heating resistor thin film of a thin film thermal head. It has been unknown whether the sputtering target can be used for film formation of a magnetic layer of a magnetic recording medium because of its magnetic characteristics, for example.
Extensive studies have been conducted to prevent the foregoing abnormal discharge and adhesion of oxide particles onto a magnetic recording medium. By controlling the particle diameter of an oxide incorporated into a target for providing a magnetic layer, it has been found that abnormal discharge can be suppressed, and adhesion of oxide particles onto a magnetic recording medium can be prevented.
In the first aspect of the present invention, a sputtering target for film formation of the magnetic layer of a magnetic recording medium comprising at least a nonmagnetic undercoat layer, a magnetic layer, and a protective layer laminated sequentially on a nonmagnetic substrate comprises a mixture of a metal and an oxide, wherein a particle diameter of the oxide being 10 xcexcm or less.
Here, the particle diameter of the oxide may be 5 xcexcm or less.
The mixture may comprise an alloy containing at least Co and Pt, and at least one oxide selected from the group consisting of oxides of Si, Ti, Zr, Al and Cr.
In the second aspect of the present invention, a method for producing a magnetic recording medium comprising at least a nonmagnetic undercoat layer, a magnetic layer, and a protective layer laminated sequentially on a nonmagnetic substrate, comprises the step of forming the magnetic layer by RF sputtering of a sputtering target for the magnetic recording medium, wherein the sputtering target for the magnetic recording medium comprises a mixture of a metal and an oxide, and a particle diameter of the oxide in the sputtering target is 10 xcexcm or less.
Here the particle diameter of the oxide in the sputtering target is may be 5 xcexcm or less.
The sputtering target may be a mixture comprising an alloy containing at least Co and Pt, and at least one oxide selected from the group consisting of oxides of Si, Ti, Zr, Al and Cr.
In the third aspect of the present invention, a magnetic recording medium comprising at least a nonmagnetic undercoat layer, a magnetic layer, and a protective layer laminated sequentially on a nonmagnetic substrate, where the magnetic layer is a granular magnetic layer having a structure in which crystal grains containing Co and having ferromagnetism are surrounded with oxide grain boundaries, the magnetic layer has been obtained by RF sputtering of a sputtering target for a magnetic recording medium, the sputtering target comprising a mixture of a metal and an oxide, the oxide having a particle diameter of 10 xcexcm or less, and defects attributed to particles of the oxide and measuring 0.05 xcexcm or more are not present on a surface of the magnetic recording medium.
According to the present invention, a magnetic layer is formed by RF sputtering of the above-described sputtering target, whereby a granular magnetic layer having excellent magnetic characteristics and electromagnetic conversion characteristics can be formed by a simple process. Furthermore, the invention is free from the problem of adhesion of oxide particles onto a magnetic recording medium.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.